It has been long known how a torque sensor may be designed in order to measure torsional moments on shafts in engines, for instance in vehicles. The torque sensor may provide a contactless measurement by means of, for instance, utilizing a transductor of a magneto-elastic type which is symmetrically shaped around a shaft, i.e. an inductive measurement. Other ways of measuring torsional moments may, for instance, be through strain sensors (for instance through trailing contacts and telemetry, i.e. resistively, capacitive measurement, optical measurement, transformer technique (transfer from primary to secondary winding), telemetry or a torductor (measure the change in the magnetic properties of a shaft).
One way of measuring torque on a shaft by using a torque sensor of a magneto-elastic type which is symmetrically designed around the shaft is previously known from U.S. Pat. No. 4,873,874. The torque sensor is furnished with three stationary coils, connected in series, in order to magnetize the shaft, and three further coils in order to detect the transferred torque in the shaft. Furthermore, the shaft is provided with three parallel, annular zones, designed in such a way that the magnetic field in the shaft may be deflected by at least 45.degree. in order to obtain good sensitivity of the torque sensor. An advantage with this torque sensor is that it reduces the sensitivity to temperature gradients in a reliable way.
Certain problems may, however, occur when mounting a sensor according to the solution in U.S. Pat. No. 4,873,874, since the mounting requires high precision and is thus work intensive. Furthermore, unnecessary wear of the zones occurs should the shaft not be sufficiently centered in relation to the coils.